Electroless (ELESS) CoWPB alloy deposition methods are under intensive investigation for providing capping layers for Cu metallization to improve Cu electromigration time in Ultra Large Scale Integration (ULSI) applications. Capping layers, as are generally well-known, may be utilized to prevent diffusion of conductive materials into subsequent layers. Despite the utility of a capping layer, without adequate process control, capping layer deposition may have undesirable results. For example, if too much capping layer material is deposited on narrow serpentine or comb structure patterns, undesirable lateral growth and mushroom formation may occur thus potentially causing increases in current leakage and electrical shorts. Typically, thin film CoWPB depositions having a thickness of less than approximately 100 Å may be required for 45 nm or 32 nm node applications. In order to achieve uniform CoWPB film depositions within required tolerances, improving ELESS plating processes and process chemistries may be desirable.
For example, one reason for the interest in CoWPB alloy deposition is due to the alloy's characteristic deposition selectivity. It is generally accepted that deposition selectivity is at least partially achieved by self-initiating autocatalytic deposition on patterned metal surfaces over dielectric surfaces. By utilizing CoWPB alloys, ELESS plating processes may be selectively enhanced on conductive surfaces. However, in some conventional solutions, nucleation sites located on dielectric surfaces may cause undesirable deposition of capping layer materials on dielectric surfaces. Thus, improving ELESS plating processes may be useful.
In another example, in some conventional applications, ELESS CoWPB plating solutions can decompose spontaneously with an increase in the volume of hydrogen gas evolution as indicated by an appearance of a black precipitate in a bulk solution. Stabilizers may be utilized in ELESS process chemistries to prevent spontaneous decomposition and uncontrollable metal precipitation. Conventional stabilizers may be divided into the following classes: (1) Heavy metals such as Sn, Pb, Cd; (2) Unsaturated organic acids: Maleic acid, itaconic acid; (3) Compound containing oxygen: Ca(SO4)2, Mo(SO4)2. However, conventional stabilizers may not, in all cases, provide adequate stabilization of ELESS plating solutions. Thus, improving ELESS process chemistries may be useful.
In still another example, in some conventional applications, bubbles may form on reactive surfaces which may otherwise lead to non-uniform deposition. Surfactants may be utilized in ELESS process chemistries to reduce the surface tension between the substrate and ELESS plating solutions to prevent bubbles from forming on the surface, which could otherwise lead to non-uniform deposition. Again, improving ELESS process chemistries may be useful.
As such, methods for improving selectivity of electroless deposition processes are provided herein.